The final mistake.
33. a4a5 Excellent play. Black doesn't have many useful moves, so White simply pushes the pawn forward. 33... d8d7 After this move, the situation becomes even more difficult. 34. c7c5 White is planning Qa3. If the queens are exchanged, the win is simple. 34... d7b7  35. a5a6 This gives Black hopes again.